Rainee N. Simons, Dale A. Force, and Thomas J. Kacpura

Glenn Research Center, Cleveland, Ohio

High Effi ciency Traveling-Wave Tube Power

Amplifi er for Ka-Band Software Defi ned Radio

on International Space Station—A Platform

for Communications Technology Development

NASA/TM—2013-217856

April 2013
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Rainee N. Simons, Dale A. Force, and Thomas J. Kacpura

Glenn Research Center, Cleveland, Ohio

High Effi ciency Traveling-Wave Tube Power

Amplifi er for Ka-Band Software Defi ned Radio

on International Space Station—A Platform

for Communications Technology Development

NASA/TM—2013-217856

April 2013

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

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NASA/TM—2013-217856 1

High Efficiency Traveling-Wave Tube Power Amplifier for Ka-Band 

Software Defined Radio on International Space Station—A Platform

for Communications Technology Development

Rainee N. Simons, Dale A. Force, and Thomas J. Kacpura

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Abstract

The design, fabrication and RF performance of the output 

traveling-wave tube amplifier (TWTA) for a space based Kaband software defined radio (SDR) is presented. The TWTA, 

the SDR and the supporting avionics are integrated to forms a 

testbed, which is currently located on an exterior truss of the 

International Space Station (ISS). The SDR in the testbed 

communicates at Ka-band frequencies through a high-gain 

antenna directed to NASA’s Tracking and Data Relay Satellite 

System (TDRSS), which communicates to the ground station 

located at White Sands Complex. The application of the 

testbed is for demonstrating new waveforms and software 

designed to enhance data delivery from scientific spacecraft 

and, the waveforms and software can be upgraded and 

reconfigured from the ground. The construction and the salient 

features of the Ka-band SDR are discussed. The testbed is 

currently undergoing on-orbit checkout and commissioning 

and is expected to operate for 3 to 5 years in space.

Introduction

The International Space Station (ISS) developed by NASA 

is one of the largest international under taking in history. It has 

brought together an international coalition of engineers and 

scientists with a common goal to pursue scientific endeavors

in space and to leverage the ISS as a springboard for future 

exploration and scientific and industrial research. NASA’s

Space Communications and Navigation (SCaN) program is

responsible for providing communications and navigation 

services to space flight missions throughout the solar system. 

In this context, NASA SCaN has recently installed a testbed 

on the exterior truss of the ISS (Ref. 1). The SCaN testbed is 

an advanced integrated communications system and laboratory 

facility. Using a new generation of Software Defined Radio 

(SDR) technologies, this ISS facility will allow researchers to 

develop, test, and demonstrate new communications, 

networking, and navigation capabilities in the actual 

environment of space (Ref. 2). The SCaN testbed will advance 

space communication technologies in support of future NASA 

missions.

The paper presents, the design, fabrication and RF 

performance of the output traveling-wave tube amplifier

(TWTA) for a space based Ka-band SDR. The TWTA, SDR,

and the supporting avionics are integrated to form the testbed, 

which is currently located on the exterior truss of the ISS. The 

SDR is NASA’s first space qualified new generation of Kaband software-reconfigurable transceiver in space. Reconfigurability provides the capability to change the 

functionality of the on-orbit radio during a mission. This 

allows optimizing the data throughput. In addition, it reduces 

the development cost and risk through reuse of a common reconfigurable space platform to meet specific mission 

requirements. Furthermore, re-configurability offers the 

flexibility to adapt to a new science mission after the primary 

mission is completed. Moreover, it allows recovery from 

anomalies within the science payload or communications 

system should they occur. The Ka-band SDR communicates 

through a high-gain antenna directed to NASA’s Tracking and 

Data Relay Satellite System (TDRSS), which then 

communicates to NASA’s ground station located at White 

Sands Complex (WSC), New Mexico. Another application of 

the testbed is for demonstrating new waveforms and software 

designed to enhance data delivery from scientific spacecraft 

and, the waveforms and software can be upgraded and 

reconfigured from the ground. Lastly, the paper presents the 

construction and the salient features of the Ka-band SDR. The 

testbed is currently undergoing on-orbit checkout and

commissioning and is expected to operate for 3 to 5 years in 

space.

High Efficiency Space Traveling-Wave 

Tube Amplifier Design and Fabrication

The helical TWTAs have demonstrated high on-orbit 

reliability at microwave frequencies (Ref. 3). In addition, 

TWTAs can deliver much higher RF output power with higher 

efficiency at Ka-Band frequencies than monolithic microwave 

integrated circuit (MMIC) based solid-state power amplifiers 

(SSPAs) (Ref. 4). Furthermore, TWTAs are capable of

handling spectrally efficient modulation techniques and very 

high data rates on the order of multi-Gbps at Ka-band 

frequencies (Ref. 5).

The baseline design for the SCaN testbed TWTA was the 

model 999HA Ka-band TWT (Ref. 6). Modern electromagnetic simulation/optimization software tools enabled the first 

pass design success. These software tools include the U.S. 

Naval Research Laboratory’s CHRISTINE 3-D Code for high 
NASA/TM—2013-217856 2

efficiency slow-wave interaction circuit and MICHELLE 3-D 

Code for multi-stage depressed collector design (Refs. 7 to 9). 

In addition, thermal modeling/simulation tools enabled the 

design of an efficient conduction cooled package (Ref. 6), 

which enhanced the power handling capability of TWT. 

Furthermore, advances in materials technology resulted in 

lightweight, temperature stable, high

B-H energy product samarium cobalt permanent magnets, 

which enabled focusing the electron beam. Moreover, 

advances in tungsten/osmium cathode technology resulted in 

cathode lifetimes exceeding 20 years in space.

The TWT has a four-stage collector circuit for high 

efficiency. The collector circuit requires high voltages, which 

are provided by a standalone 7-kV EPC, which is attached by 

an umbilical power cord to the TWT. The EPC is designed to 

operate from an unregulated spacecraft bus voltage in the 

range of 21.25 to 35 V. The EPC has also a telemetry interface 

to the spacecraft bus, which provides the ON/OFF state of the 

TWTA as well as the anode voltage, helix current and the RF 

output power. The construction of the EPC is explained in 

Reference 6. 

SCaN Testbed Traveling-Wave Tube 

Amplifier Performance Characteristics

The TWTA was characterized over the frequencies 25.5 to 

25.8 GHz reserved for NASA’s near-Earth communications. 

The measured data reported here are at a nominal spacecraft 

bus voltage of 31 V and ambient temperature of 25 °C.

RF Output Power and Phase Shift 

The measured relative RF output power and phase shift as a 

function of the input drive level is presented in Figure 1.

These measurements were carried out at the center frequency 

of 25.65 GHz. The output power at saturation is 46.22 dB or 

41.88 W, which exceeds the requirements of 40 W. The phase 

shift through the amplifier, as the input drive is increased from 

small signal to saturation drive, is about 30°. The saturated 

gain compression is 6.09 dB and the saturated AM-to-PM 

conversion is 3.5°/dB.

Saturated RF Gain, Saturated Output Power 

and Overall Efficiency

The measured saturated RF gain is about 49.25 dB across 

the operating frequency range and exceeds the requirements of 

46 dB. The measured saturated RF output power is about 

46.25 dB or 42.17 W across the frequency range and exceeds 

the end of life (EOL) requirements of 40 W. The efficiency of 

the TWT alone is 55.2 percent. However, when the 

efficiencies of the TWT and the EPC are combined, the 

Figure 1.—Measured TWT relative RF output power and phase 

shift as a function of the RF input drive relative to saturation 

at the center frequency of 25.65 GHz.

Figure 2.—Measured overall TWTA efficiency as a function of 

frequency.

overall efficiency of the TWTA is about 45 percent. The

measured overall efficiency as a function of the operating 

frequency range is presented in Figure 2. The overall 

efficiency exceeds the requirements of 44 percent.

Noise Figure, Second Harmonic Level, Small 

Signal Gain, Phase Shift and Input/Output 

VSWR

The measurements made at the center frequency of

25.65 GHz show that the noise figure is 29.97 dB and the 

second harmonic level is 23.52 dB below the fundamental. 

The measured small signal gain ripple is 0.46 dB peak-topeak, which is within the requirements. The residual peak-topeak small signal phase after subtracting the linear phase 

component corresponding to a group delay of 3.798 ns is 

2.02°, which is also within the requirements. The measured 

input/output VSWR is in the range of 1.4 to 1.7, over the 

operating frequency range of 25.5 to 25.8 GHz, which is 

within the requirements.
NASA/TM—2013-217856 3

Ka-Band Re-Configurable Software 

Defined Radio

Harris Corporation developed the Ka-band software defined 

radio (SDR). The SDR consists of a single board computer, 

modem boards, digital IO boards, sampler card, transmit/

receive RF circuits, master oscillator and power supply. The 

SDR is illustrated in Figure 3. The SDR transmit center 

frequencies is 25.65 GHz and the receive center frequency is 

22.6795 GHz. These frequencies are located in the band 

designated for TDRSS Ka-band operation. The transmit data 

rate is variable from 300 kbps to 100 Mbps, the modulation is 

offset-QPSK with rate one-half convolution coding. The 

receive data rate is variable from 300 kbps to 25 Mbps, the 

modulation is BPSK with rate one-half convolution coding.

Ka-Band Scan Testbed on ISS

The major microwave components of the testbed are the 

TWTA, the isolator, the diplexer, the gimbal assembly and the 

high gain antenna. The layout is shown in Figure 4. The high 

gain antenna is a 0.5 m diameter parabolic reflector with the 

feed horn placed at the focus. The antenna is designed to 

support TDRSS Ka-band communications. The Sierra Nevada 

Corporation provided the integrated antenna pointing system, 

which features an open-loop microstepping technology. The 

testbed avionics unit provides the DC power, handles the 

command and control signals and the telemetry. The SDR on 

the SCaN testbed payload, illustrated in Figure 5,

communicates with the TDRSS over a full-duplex Ka-band 

link. This feature contrasts with prior NASA missions, such as 

the Lunar Reconnaissance Orbiter (LRO), Mars Reconnaissance 

Orbiter (MRO) and the Solar Dynamic Orbiter (SDO), which 

used Ka-band for communications; however, the Ka-band 

radios on these missions were transmit only.

Figure 3.—Ka-band software defined radio hardware.

Figure 4.—Ka-band ScaN testbed flight system connections. 

The data, command and telemetry links are using SpaceWire 

standard.

Figure 5.—SCaN testbed payload flight unit concept.
NASA/TM—2013-217856 4

Conclusions and Discussions

The performance parameters of a state-of-the-art compact 

lightweight Ka-band TWTA for NASA’s SCaN testbed is 

presented. The saturated RF output power exceeds 40 W and 

saturated RF gain exceeds 46 dB. The overall efficiency 

including that of the EPC is as high as 45 percent. The Harris 

SDR is NASA’s first space qualified Ka-band SDR. The SDR

and the TWTA have been integrated into the SCaN testbed

and will support communications with NASA’s TDRSS. The 

re-configurability of the SDR promotes experiments, which 

supports the next generation mission concepts and future 

science missions. Prior to launch the testbed successfully 

completed the system level random vibration, EMI/EMC, 

thermal vacuum, and TDRSS compatibility tests. The SCaN 

testbed was launched in July 2012 to the ISS on a Japanese

H-II Transfer Vehicle (HTV-3) and is now located on the 

exterior truss of the ISS. The SCaN testbed is currently 

undergoing on-orbit checkout and commissioning, which will 

be followed by a multi-year experiment period starting mid-2013.

References

1. http://spaceflightsystems.grc.nasa.gov/SOPO/SCO/SCaN

Testbed/

2. S.K. Johnson, R.C. Reinhart, and T.J. Kacpura, 

“CoNNeCT’s approach for the development of three 

Software Defined Radios for space application,” 2012 

IEEE Aerospace Conference, pp. 1-13, Big Sky, MT, 

March 3-10, 2012.

3. E.F. Nicol, B.J. Mangus, and J.R. Grebliunas, “TWTA 

versus SSPA: Analysis update of the Boeing fleet on-orbit 

reliability data,” 2008 IEEE International Vacuum 

Electronics Conference Digest, pp. 169–170, Monterey, 

CA, April 22–24, 2008.

4. R.N. Simons, J.D. Wilson, and D.A. Force, “High power 

and efficiency space traveling-wave tube amplifiers with 

reduced size and mass for NASA missions,” 2008 IEEE 

MTT-S International Microwave Symposium Digest, pp. 

319–322, Atlanta, GA, June 15–20, 2008. 

5. R.N. Simons, E.G. Wintucky, D.G. Landon, J.Y. Sun, J.S. 

Winn, S. Laraway, W.K. McIntire, J.L. Metz, F.J. Smith, 

“Demonstration of Multi-Gbps Data Rates at Ka-Band 

Using Software-Defined Modem and Broadband High 

Power Amplifier for Space Communications,” 2011 IEEE 

MTT-S International Microwave Symposium Digest, pp. 

1–4, Baltimore, MD, June 5–10, 2011.

6. W.L. Menninger, N.R. Robbins, D.R. Dibb, and D.E. 

Lewis, “Power flexible Ka-band traveling wave tube 

amplifiers of up to 250-W RF for space communications,” 

IEEE Trans. Electron Devices, vol. 54, no. 2, pp. 181–

187, Feb. 2007.

7. D. Chernin, T.M. Antonsen, Jr., B. Levush, and D.R. 

Whaley, “A three-dimensional multifrequency large 

signal model for helix traveling wave tubes,” IEEE Trans. 

Electron Devices, vol. 48, no. 1, pp. 3–11, Jan. 2001.

8. D.K. Abe, B. Levush, T.M. Antonsen, Jr., D.R. Whaley 

and B.G. Danly, “Design of a linear C-band helix TWT 

for digital communications experiments using the 

CHRISTINE suite of large-signal codes,” IEEE Trans. 

Plasma Science, vol. 30, no. 3, part 1, pp. 1053–1062, 

June 2002.

9. J.J. Petillo, E.M. Nelson, J.F. DeFord, N.J. Dionne, and B. 

Levush, “Recent developments to the MICHELLE 2-D/3-

D electron gun and collector modeling code,” IEEE 

Trans. Electron Devices, vol. 52, no. 5, pp. 742–748, May 

2005.
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High Efficiency Traveling-Wave Tube Power Amplifier for Ka-Band Software Defined 

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14. ABSTRACT

The design, fabrication and RF performance of the output traveling-wave tube amplifier (TWTA) for a space based Ka-band software 

defined radio (SDR) is presented. The TWTA, the SDR and the supporting avionics are integrated to forms a testbed, which is currently 

located on an exterior truss of the International Space Station (ISS). The SDR in the testbed communicates at Ka-band frequencies through a 

high-gain antenna directed to NASA’s Tracking and Data Relay Satellite System (TDRSS), which communicates to the ground station

located at White Sands Complex. The application of the testbed is for demonstrating new waveforms and software designed to enhance data 

delivery from scientific spacecraft and, the waveforms and software can be upgraded and reconfigured from the ground. The construction 

and the salient features of the Ka-band SDR are discussed. The testbed is currently undergoing on-orbit checkout and commissioning and is 

expected to operate for 3 to 5 years in space. 

15. SUBJECT TERMS

Telecommunications; Transmitters; Deep Space Network; Space communication; Satellite communications; Microwave transmission; 

Microwave amplifiers; Microwave tubes; Power amplifiers; Power conditioning; Power efficiency; Traveling wave tubes; Radio 

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